Roof Area From Plan Calculator

Purpose and model boundary

This calculator estimates the roofing to order for a roof you draw. You draw the roof as seen from above on a grid: its outline, then its ridges, hips and valleys, which divide it into facets (the flat planes of the roof), and you set each facet's pitch. The spreadsheet reads the drawing's edges and facets, works out which way each facet slopes, adds the overhangs, and returns the true, sloped roof area, the roofing squares, the shingle bundles and underlayment rolls to order, the length of starter strip, drip edge and ridge cap, the length of every kind of roof line, a suggested waste allowance, and each facet's area in a table. It is not a roof design, a survey or a cost estimate: it does not size rafters or check loads, and flashing, vents, fasteners, labour and prices are not estimated.

Inputs and units

The drawing is the first input. Its outline is sent as an ordered list of up to 100 corner points, as on the Area Calculator. With Roof, the drawing tool marks each outline piece as an eave, a rake, a high side, a headwall or a sidewall, and draws the roof lines inside it: ridges, hips, valleys, pitch changes and the wall lines where a lower roof meets a wall (a headwall or a sidewall). Roof shape draws a whole gable or hip roof at once, and Set pitch gives a facet its pitch as a rise in 12, in degrees or as a percent. The drawing sends three tables: the grid step, every roof edge (its ID, its type, its two ends and the facet on each side of it), and every facet with its pitch as typed. It sends no length, area, rise or slope direction: the spreadsheet computes them all from the edges. Up to five exclusions of up to 20 points each, such as a chimney or a skylight, may be drawn too (Add exclusion), but they are not taken off the roof, which is laid over them. The US/Metric switch on the drawing chooses the unit the points and edges are written in, feet for US and metres for Metric, and the unit of every result that has one; it never converts a pitch.

To draw a complex roof, draw its outline and choose Roof ▾, Roof shape ▸, Generate roof: it draws every ridge, hip and valley of an outline whose corners are all square at once, or says why it cannot. A gable end is Mark edges ▸ Rake on that outline piece, and a wing's pitch is Set pitch on its facets, then Replace with generated roof keeping each facet's pitch.

The inputs beside the drawing are:

• Outline drawn at: where the outline you drew sits. Walls (the default): the eave and rake overhangs are added outside it, and each dormer or porch wall line is a wall, with the upper roof's overhang past it added to the upper roof. Roof edge: the outline is the drip line and nothing is added, and each wall line is the upper roof's edge, with the lower roof running under it by one overhang added to the lower roof.
• Pitch where the drawing sets none (rise in 12): the pitch used for a facet the drawing gives no pitch, from 0 to 36 in 12. It is the same figure in both systems. The page opens on one facet with no pitch, so until you set each facet's pitch this one pitch is used over the whole plan, and the status says so.
• Eave overhang: how far the roof runs past the walls at the eaves and high sides, in feet (US) or metres (Metric).
• Rake overhang: how far the roof runs past the walls at the rakes (the sloped gable edges), in feet or metres. Headwalls and sidewalls take no overhang.
• Roofing waste: the extra shingles and underlayment for cutting and overlaps, as a percentage of the true roof area, from 0% to 100%.
• Bundles per square: how many shingle bundles cover one roofing square of 100 sq ft; a whole number of at least one.
• Underlayment roll coverage: the roof area one roll covers, in square feet (US) or square metres (Metric).

The two overhangs and the roll coverage have one input for each system, each with its own default: 1 ft, 1 ft and 400 sq ft in US, and 0.5 m, 0.5 m and 37 m² in Metric. The two sets are the figures of two different worked examples, not conversions of each other: 1 ft is 0.3048 m, not 0.5, and 400 sq ft is 37.16 m², not 37. The outline setting (Walls), the pitch (6 in 12), the waste (10%) and the bundles per square (3) are shared by both systems. Switching between US and Metric shows the other system's inputs with their own values and never converts a number you typed; the value you typed for the system you left is kept for when you switch back, and it is not sent while it is hidden.

The defaults are the figures of the worked examples this calculator was built against, not values from a code, a standard or a manufacturer, so review them before use. Check your shingle's wrapper for the bundles per square and the roll for its coverage.

Governing relationships

The plan area is the area inside the drawn outline, measured by the shoelace formula as on the Area Calculator. Each facet's plan area is measured the same way from its own edges, their points read to the nearest 25 micrometres (see Validation and status logic), and the facets must cover the outline exactly: a request whose facets and outline disagree is refused.

Which way a facet slopes comes from its level edges: it slopes down toward an eave and away from a ridge, a high side, a headwall or the upper side of a pitch change. Its slope is its pitch as a rise over run: a rise in 12 of p is p ÷ 12, an angle of d degrees is tan(d), and a percent of q is q ÷ 100. Every edge's rise follows from the slope and the facet's downhill direction, taken from each facet it bounds; a hip or valley between two facets gets its rise from both sides, so hips and valleys between different pitches are measured where the drawing puts them, and the two sides must agree.

Level edges (eaves, high sides, headwalls, ridges and pitch changes) are measured at their plan length. Sloped edges (rakes, sidewalls, hips and valleys) are measured at their true length from their plan length and rise: True length = √(plan length² + rise²), a hip or valley with the average of its two sides' rises.

The overhang, when the outline is drawn at the walls, moves each outline piece out by its own overhang (the eave overhang at an eave or high side, the rake overhang at a rake, none at a headwall or sidewall), and the moved pieces meet at new corners, mitred: each is where the two moved pieces, extended, meet. Where two pieces with different overhangs continue in one straight line, the moved pieces do not meet, and the overhang steps out between them at the drawn corner. Each new corner is taken from the two outline pieces that share the drawn corner, whatever order the drawing lists them in, so an outline drawn clockwise gives the same figures as one drawn anticlockwise. Each piece's strip of overhang is the trapezoid between the piece and its moved line, added to the facet the piece bounds; a ridge or pitch change that ends at an outline corner, and a hip or valley between two equal pitches, is lengthened to that corner's moved point, at its own grade. A hip or valley between two different pitches is lengthened along its own line instead, to where the two planes' crease reaches the nearer of the two moved lines: t = min(d₁ ÷ (c · o₁), d₂ ÷ (c · o₂)) past the corner, where c is the line's direction, o₁ and o₂ the two outline pieces' outward directions and d₁ and d₂ their overhangs. The small triangle between that crease and the line to the moved corner goes to the facet on the moved corner's side of the crease. Each plane continues to its own fascia, so where two different pitches meet at a corner, their fascias meet at different heights. Each wall line adds the upper roof's overhang past it along its length: to the upper roof when the outline is drawn at the walls, to the lower roof when it is drawn at the roof edge (the drip line). When the outline is drawn at the roof edge, nothing else is added.

The pitch turns each facet's plan area into its sloped area:

Facet area on the slope = facet plan area with its overhang × √(1 + slope²)

True roof area = the sum of every facet's area on the slope

Pitch factor = true roof area ÷ plan area with overhang, an average weighted by area

Squares = true roof area ÷ 100, in square feet (US), or ÷ 9.290304, in square metres (Metric). A roofing square is 100 sq ft in both systems, and 9.290304 m² is 100 sq ft converted exactly. The squares are before waste.

Shingle bundles = squares × (1 + waste) × bundles per square, rounded up to a whole bundle

Underlayment rolls = true roof area × (1 + waste) ÷ roll coverage, rounded up to a whole roll

Each count is rounded to six decimal places before it is rounded up, so a count that is whole but for the last binary digit of the arithmetic is not pushed to the next.

The roof lines are added up by type, each at its true length and each outline piece with its overhang; a wall line is counted twice, as a headwall or sidewall and as the upper roof's eave or rake. From them:

Starter strip = eave length

Drip edge = eaves + rakes + high sides, the rakes along the slope

Ridge cap = ridges + hips, the hips along the hip

The suggested waste is (2.3 × valleys + 0.7 × hips + 0.25 × rakes + 0.25 × sidewalls) ÷ true roof area + 5%, with the lengths in feet and the area in square feet (square feet per foot of line); in Metric the same allowance is computed from metres. It is a rule of thumb for standard architectural shingles laid with closed, single-cut valleys, and it covers the field shingles only: the starter strip and ridge cap are counted separately above. Other shingles, and woven or open valleys, need a different allowance. It is not a figure from any code, standard or manufacturer. It is shown in the Shingles results and not applied: the bundles and rolls use the waste you type.

Nothing else is rounded in the spreadsheet: every figure is computed from unrounded values.

For the drawing the page opens with, the canvas's 20 by 15 ft rectangle of 300 sq ft drawn at the walls as one facet with no pitch, the typed 6 in 12 and 1 ft overhangs give 374 sq ft in plan (the 22 by 17 ft rectangle of the eaves), 374 × √(1 + 0.5²) = 418.14 sq ft on the slope, 4.18 squares, 14 bundles, 2 rolls and 78 ft of eaves, so 78 ft of starter strip and of drip edge; no ridge is drawn yet, so the ridge cap reads 0. Drawn as a gable with Roof shape (worked example 11.4.9.1), the same plan keeps its 418.14 sq ft, but its two eaves are 22 ft each (44 ft of starter strip), its four rakes, each lengthened by the eave overhang and measured on the slope, add 38.01 ft for 82.01 ft of drip edge, and its ridge, lengthened by the rake overhang at both ends, is 22 ft. Drawn as a hip roof, each hip runs from the ridge to an eave corner and on to the corner of the overhang: 12.75 ft along the hip, 51 ft for the four, and with the 5 ft ridge a 56 ft ridge cap.

Calculation sequence

1. The spreadsheet measures the plan area of the outline and the area of each exclusion. The exclusions are not taken off.
2. It reads the three roof tables, the outline setting, the eave and rake overhangs and the roll coverage of the selected system, and the typed pitch, the waste and the bundles per square.
3. It checks that every roof edge has an ID, a known type, both ends and its facets. It reads every roof point and outline point to the nearest point of a 25 micrometre grid and refuses a point that is more than 1,500 ft (450 m) from the origin along x or y once rounded to that grid. It then checks that no other edge shares an edge's ID, that each edge's two ends differ, that every facet an edge names is listed under exactly that ID, and every facet's pitch, and measures each facet's plan area from its own edges. The facets must cover the outline exactly, once.
4. Each facet's slope comes from its pitch, or from the typed pitch when the drawing gives it none, and its downhill direction from its level edges. Every edge's rise follows from them, and each edge is checked against its type and against the facets on both its sides.
5. When the outline is drawn at the walls, each outline piece is moved out by its own overhang, its strip is added to its facet, and the lines that end at an outline corner are lengthened past it: a hip or valley between two different pitches to where the two planes' crease reaches the nearer moved line, and every other line to the moved corner. The overhang is refused unless the strips fit together without overlapping or touching, within 2,500 ft (750 m) of the origin along x and y once rounded to the 25 micrometre grid (see Assumptions and limitations). The wall lines add the upper roof's overhang past them.
6. Each facet's plan area with its overhang, times its pitch factor, is its area on the slope. The plan area with overhang, the true roof area and the pitch factor, an average weighted by area, follow from their sums.
7. Squares: the true roof area over one square, 100 sq ft or 9.290304 m².
8. Bundles and rolls, each with the waste allowance, rounded to six decimal places and then up to a whole bundle or roll.
9. The roof lines are added up by type at their true lengths. Starter strip is the eaves; drip edge is the eaves, rakes and high sides; ridge cap is the ridges and hips. The suggested waste is computed from the valleys, hips, rakes and sidewalls.
10. The "Each facet" table lists every facet, in the order of the Facets table, with its pitch as set, its plan area with its overhang and its area on the slope.
11. The status line is chosen, as described below. When it begins NOT VALID:, every computed figure reads zero and the "Each facet" table is blank; the plan area and the exclusion areas are still measured from the drawing.

Outputs and interpretation

True roof area, Roofing squares, Shingle bundles, Underlayment rolls, Starter strip, Drip edge and Ridge cap are the seven headline results. The true roof area is in square feet (US) or square metres (Metric), and the lengths in feet or metres, each shown to two decimals. The squares, bundles and rolls have no unit.

Under "Roof lines" the page shows the length of each kind of line: eaves, rakes, high sides, ridges, hips, valleys, pitch changes, headwalls and sidewalls. Under "How the roof is counted", Plan area with overhang and Pitch factor are the steps the true roof area is built on. Under "From the drawing" the page shows the plan area inside the drawn outline and, so you can check the drawing, the plan area less exclusions and each exclusion's area; none of those last six is used for the roof, which covers the exclusions. After all of these, the "Each facet" table lists every facet with its pitch as set (a facet with no pitch shows the typed pitch, marked "typed"), its plan area with its overhang and its area on the slope; its headings are the spreadsheet's, in the selected system's units.

A result belongs to the drawing and the inputs it was calculated for. When you change either after pressing Calculate, the result is kept but marked stale until you press Calculate again.

Validation and status logic

The drawing tool checks every edit before it is kept: the Area Calculator's rules for the outline and exclusions, and roof mode's own (at most 40 facets and 160 roof edges, a roof line from edge to edge or node to node, a pitch from 0 to 36 in 12, 71.5 degrees or 300 percent). It always writes the tables in consecutive rows, every edge with its type and its facets. The spreadsheet checks the tables again, because a request need not come from the drawing.

The status line is the spreadsheet's own verdict, the first of these that applies. Where a line names a facet (<facet>), a roof edge (<edge>), a line's type (<type>) or a figure, the spreadsheet puts in the first one in the list; <more> is "1 more facet" or the number of further facets, <x> and <y> are where a corner sits and <length> a length, each to two decimals, except that a point past the bound is written as the drawing sent it, to 15 significant digits; <number> is an outline point's place in the list, <bound> is 1,500 ft or 450 m, <limit> is 2,500 ft or 750 m, and <unit> is ft or m:

• NOT VALID: choose US or Metric
• NOT VALID: outline and exclusion points must fill consecutive rows, with no blank row between them
• NOT VALID: draw at least three outline points
• NOT VALID: every outline point needs both coordinates
• NOT VALID: every point of exclusion 1 needs both coordinates
• NOT VALID: every point of exclusion 2 needs both coordinates
• NOT VALID: every point of exclusion 3 needs both coordinates
• NOT VALID: every point of exclusion 4 needs both coordinates
• NOT VALID: every point of exclusion 5 needs both coordinates
• NOT VALID: each exclusion needs zero or at least three complete points
• NOT VALID: the roof outline encloses no area
• NOT VALID: the outline repeats a point or turns straight back on itself
• NOT VALID: the roof edges and facets are missing; draw the roof on the page's FloorCanvas
• NOT VALID: roof edges and facets must fill consecutive rows, with no blank row between them
• NOT VALID: every roof edge needs an ID, a known type, both ends and its facets
• NOT VALID: roof edge <edge> has a point, (<x>, <y> <unit>), more than <bound> from the origin along x or y; draw the roof again on the page
• NOT VALID: outline point <number>, (<x>, <y> <unit>), is more than <bound> from the origin along x or y; draw the outline again on the page
• NOT VALID: roof edge ID <edge> is given to more than one row; give each roof edge its own ID, or draw the roof again on the page
• NOT VALID: roof edge <edge> starts and ends at the same point; delete it or draw the roof again on the page
• NOT VALID: roof edge <edge> names a facet the Facets table does not list
• NOT VALID: the pitch of <facet> must be a number with its unit (rise/12 up to 36, degrees up to 71.5, percent up to 300)
• NOT VALID: facet <facet> is listed in Facets but no roof edge bounds it; delete it or draw the roof again on the page
• NOT VALID: a <type> inside <facet> ends in the open; join it or delete it (<edge>)
• NOT VALID: <facet> has no level edge (an eave, ridge, high side, headwall or pitch change) to set which way it slopes
• NOT VALID: the facets do not cover the drawn outline; draw the roof again on the page
• NOT VALID: the edges of <facet> do not close into a loop at (<x>, <y> <unit>); draw the roof again on the page
• NOT VALID: roof edges <edge> and <edge> are the same line drawn twice, so their facets overlap; draw the roof again on the page
• NOT VALID: roof edge <edge> crosses, overlaps or touches another roof edge away from their shared corners, so facets overlap; draw the roof again on the page
• NOT VALID: the outline pieces total <length> <unit> but the drawn outline's perimeter is <length> <unit>; the facets must cover the outline exactly once; draw the roof again on the page
• NOT VALID: outline piece <edge> does not lie on the drawn outline; draw the roof again on the page
• NOT VALID: <facet> is not on the side of roof edge <edge> that the edge names; draw the roof again on the page
• NOT VALID: the outline touches itself at (<x>, <y> <unit>); draw the roof again on the page with an outline that does not touch itself
• NOT VALID: the outline crosses or touches itself on its side from (<x>, <y>) to (<x>, <y> <unit>); draw the outline again on the page so that it does not cross or touch itself
• NOT VALID: enter the pitch used where the drawing sets none, from 0 to 36 in 12
• NOT VALID: enter eave and rake overhangs of zero or more
• NOT VALID: the overhangs meet across a narrow recess in the outline; draw the outline straight across that recess, since the roof covers it
• NOT VALID: the overhang at roof edge <edge> reaches more than <limit> from the origin along x or y; reduce the overhang, or draw the outline at the roof edge
• NOT VALID: the overhang overlaps itself across a recess near roof edge <edge>; reduce the overhang, or draw the outline at the roof edge
• NOT VALID: enter a roofing waste between 0% and 100%
• NOT VALID: bundles per square must be a whole number of at least one
• NOT VALID: underlayment roll coverage must be greater than zero
• CHECK: <facet> has no pitch on the drawing, so the pitch typed here (<pitch> in 12) is used; set each facet's pitch with Set pitch
• CHECK: <facet> and <more> have no pitch on the drawing, so the pitch typed here (<pitch> in 12) is used; set each facet's pitch with Set pitch
• CHECK: The roof drawing has unresolved or inconsistent facets (see the notes under the drawing); hip and valley lengths may be wrong.
• CHECK: the level edges of <facet> do not agree on which way it slopes; check the type of <edge>
• CHECK: <edge>, marked <type>, does not fit the slope of <facet>; check its type or the facet's pitch
• CHECK: <facet> and <facet> give <type> <edge> different rises (<rise> and <rise> <unit>); check their pitches or where it is drawn
• CHECK: <facet> and <facet> meet at different heights along <edge> (<height> and <height> <unit>); check their pitches, or whether their eaves are at one level
• CHECK: <facet> and <facet> have different pitches where <type> <edge> reaches the overhang, but it does not run out of that corner between their two outline pieces, or another such line ends there, so its length and their areas past the wall are approximate; draw the outline at the roof edge for exact figures
• CHECK: the exclusions you drew are not taken off; the roof figures cover the whole outline, as the requirement estimates a roof
• OK

A facet with no pitch is not refused: the pitch you type is used for it and the status says which facet, so the page's opening drawing computes at once. The rises of a hip or valley are compared from both of its facets, within half an inch (1 cm) plus the drawing's grid step times the steeper slope; a gable end left as an eave, a hip drawn where the planes do not meet, or a ridge off the line where two planes meet each raises a CHECK: that names the edge, because the figures are computed but the drawing does not describe one roof. Where a hip or valley between different pitches ends at an outline corner, the areas and the hip and valley lengths are exact at the walls too; the eave, rake and high-side lengths there, and so the starter strip and the drip edge, still follow the moved outline to the moved corner, which is not the fascias' own length. Past the wall, the two planes' crease runs on along the line until it reaches the nearer fascia, and each plane continues to its own fascia. A CHECK says so only where that cannot be traced: the line does not run out of its corner between its two facets' outline pieces, or a second such line ends at the same corner. That CHECK is defensive: a drawing whose hips and valleys rise the same from both sides never reaches it. A headwall or sidewall at such a corner is covered, since it has no overhang: the line stops at the wall. A CHECK: verdict comes with the figures; a NOT VALID: verdict is shown in place of them, and every computed figure reads zero while the drawn areas still show.

The roof checks are exact. Every roof point and every outline point is first read to the nearest point of a 25 micrometre grid, 12,192 steps to the foot or 40,000 to the metre. Every point is read at the nearest point of a 25 micrometre grid, at most about 17.7 micrometres from the value the page sends (12.5 along x and along y), so a point on the drawing tool's 1/8 in, 1 mm or 0.5 mm grid is read exactly. A drawing converted by the page's US/Metric switch is read the same way. A point that is more than 1,500 ft (450 m) from the origin along x or y once rounded to that grid is refused and named as the drawing sent it, to 15 significant digits; within that bound the spreadsheet's arithmetic on the grid points is exact. Every roof figure is computed from the grid points; the plan area, the plan area less exclusions and the exclusion areas are measured from the points as sent. Facets are named by their exact IDs: letter case counts, and *, ? and ~ are ordinary characters, so a line whose facet is named F* names no facet unless one is listed as F*. On the grid every test that decides whether the facets cover the outline is done in whole numbers, with no tolerance and no exception near a corner: two corners are the same only when their grid points are, two roof edges may meet only at a shared end (a crossing, an end on another edge's interior, such as an unsplit T-junction, or a line along another is refused), each outline piece must lie exactly on one side of the outline, each facet is read just to its side of the middle of each of its edges, and the outline's own sides may not cross or touch. Where a roof line lands inside a side of the outline, the drawing tool writes that point into the outline too, so the outline and the roof line are read at the same grid point. Together with the loop, drawn-twice and naming checks, these prove that the facets partition the outline as read on the grid: no two overlap and together they leave no gap, so each facet's area is its share of that outline's. That outline differs from the one sent only where a point was off the grid, and its area by at most about the perimeter times 18 micrometres (half a step's diagonal). The area sum, the perimeter total, the corner-touching item and the 1/2 in (1 cm) touching item are further refusals the proof does not need. The first two and the last keep their tolerances; the area sum allows a millionth of the plan area (of one square foot or square metre, if that is larger) plus the perimeter times 0.1 mm, because the plan area is measured from the points as sent.

Assumptions and limitations

• The heights of the facets assume every outline eave is at one level; the check on meeting heights says when they are not.
• A facet slopes one way, set by its level edges; a curved or warped roof is drawn as flat facets.
• The overhang is added with mitred corners: the moved pieces are extended until they meet, so at a sharp corner the overhang reaches well past the drawn corner. The overhang strips must fit the band between the outline and its moved outline exactly, each part of it covered once. Overhangs that overlap, or meet across a recess in the outline, are refused: a piece's moved side shrinks to nothing or reverses, the moved outline crosses or touches itself, or one of its corners comes within 50 micrometres of a side it is not on. A moved outline more than 2,500 ft (750 m) from the origin along x or y, once rounded to the 25 micrometre grid, is refused too. Reduce the overhang, draw the outline straight across the recess (the roof covers it), or draw the outline at the roof edge.
• Where a hip or valley between two different pitches meets the overhang at the walls, each plane continues to its own fascia, so the two fascias there meet at different heights and are not one level line. There the areas and the hip and valley lengths follow the two planes' crease, while the eave, rake and high-side lengths, and so the starter strip and the drip edge, follow the moved outline to the moved corner, which is not the fascias' own length. Where that crease cannot be traced past the wall, the corner keeps the mitre, which is approximate, and the status says CHECK. Between equal pitches the overhang corner is the plain mitre, as before.
• The starter strip is the eaves only and the drip edge the eaves, rakes and high sides. A roof whose starter or drip edge is laid elsewhere needs its own count.
• Exclusions are not taken off the roof, and they add no flashing or cricket.
• The suggested waste is a rule of thumb for standard architectural shingles with closed, single-cut valleys, for the field shingles only, shown and not applied; waste depends on the shingle, the valley method, the cutting and the installer.
• No valley underlayment, ice barrier, flashing, vents, fasteners, labour or pricing are estimated.
• The spreadsheet does not assume that the facets come from the drawing tool, and with the roof drawn it tests the outline too. It reads every roof point and outline point to the nearest 25 micrometres, checks the roof tables against the outline exactly on those points, and refuses them unless every point, once rounded to that grid, is within 1,500 ft (450 m) of the origin along x and y, every roof edge has an ID no other edge has and two different ends, every facet an edge names is listed under exactly that ID, every facet listed is bounded by some edge, the facets' plan areas sum to the outline's, each facet's lines close into a loop, no line is drawn twice or crosses, overlaps or touches another away from their shared corners, the outline pieces total its perimeter (within 1/2 in, 1 cm) and each lies exactly on one side of the outline, each facet is on the side its lines name, the outline does not touch itself at a corner (no corner where two outline pieces start, or two end), and no side of the outline crosses or touches another. It does not check the order the edges and facets are listed in. Every figure, and whether the status is OK, CHECK or NOT VALID, is independent of that order: each outline piece is paired with the pieces that share its corners, not with the rows listed next to it, and each facet's downhill direction and the eave its heights are measured from come from its reference level edge (an outline eave if it has one, else any of its level edges, the one that sorts first by its corners), not from the first one listed. The order decides only which edge or facet a status line names, the first qualifying one in the list, and the order of the "Each facet" table's rows, which follow the Facets table. The calculator is offered on its web page only, because the roof is drawn there.
• An edge that crosses another, has another's end on its interior, or passes within 1/2 in (1 cm) of another edge's midpoint is refused as touching, so a very narrow facet, about 1 in (2 cm) wide or less, can be refused. The drawing tool can draw one (on its 1 in, 1 cm or 2 cm grid, or as a corner almost on a long line at a shallow angle); draw facets wider than that.
• Drawing needs a mouse, a pen or a touch screen; a drawing prepared elsewhere can be loaded with Import.

Restrictions and non-computing states

This calculator accepts a pitch from 0 to 36 in 12, overhangs of zero or more, a waste from 0% to 100%, a whole number of bundles per square of at least one and a roll coverage greater than zero; a roll coverage of zero, overhangs that overlap or meet across a recess in the outline, and an overhang reaching more than 2,500 ft (750 m) from the origin along x or y once rounded to the 25 micrometre grid, are refused by the status line. The drawing always sends its three roof tables, so they are required: a request without them is refused before the spreadsheet is asked. An input of the system that is not selected is not read. Nothing is sent until you press Calculate, and pressing it again on an unchanged drawing with unchanged inputs sends nothing.

Errors and warnings

A blank required input keeps Calculate from sending, and pressing it names the input and moves to it. If the calculation service refuses an input, the status line names it. If the service does not answer, the page says so, keeps your drawing, your inputs and the last result, and lets you press Calculate again. A CHECK: status means the figures are returned with a reason to look again; a NOT VALID: status means they cannot be computed and none are shown.

Sources and further reading

• National Institute of Standards and Technology, Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B.8. nist.gov. The exact foot (0.3048 m) and the exact square foot, 0.09290304 m², from which a roofing square of 100 sq ft is 9.290304 m².
• Eric W. Weisstein, "Polygon Area," MathWorld, Wolfram Research. mathworld.wolfram.com/PolygonArea.html. The shoelace formula for the area of a polygon from its corners, which the spreadsheet uses for the outline and for each facet.
• Eric W. Weisstein, "Pythagorean Theorem," MathWorld, Wolfram Research. mathworld.wolfram.com/PythagoreanTheorem.html. a² + b² = c², which gives the length of a slope from its run and rise: the pitch factor, and the true length of a rake, hip or valley.
• Asphalt Roofing Manufacturers Association, Residential Asphalt Roofing Manual (2014). asphaltroofing.org. The roofing square of 100 sq ft, the measurement of a roof's area from its plan and slope, and waste allowances that grow with a roof's complexity.
• International Residential Code 2021, section R905.2.8.5, drip edge, as published by Jaspector. jaspector.com. Drip edge at the eaves and the rakes, the basis of the drip edge figure.
• Owens Corning, Starter Strip shingle installation instructions. buildsite.com. Starter strip laid along the eaves, the basis of the starter figure.

Links were current at build time. If one moves, search the document's title rather than reporting a broken link as a defect.

The default pitch, overhangs, waste, bundles per square and roll coverage are the figures of the worked examples and the requirement this calculator was built to, not values from these sources or from any code, standard or manufacturer; each system has its own. These sources describe the units, the geometry and common practice; they do not certify a particular roof or its result. Use the pitches and overhangs on your roof plans, the coverage printed on your shingles and underlayment, and check the figures before ordering.